1. Field of the Invention
The present invention relates to a light measuring apparatus for camera.
2. Description of the Prior Art
In the art there is known and used such type of light measuring circuit comprising a light receptor photo diode, a high input impedance operational amplifier and a logarithmic conversion diode. In this type of the circuit, the two ends of the photo diode is imaginarily short-circuited. The photo current generated in the photo diode flows into the logarithmic conversion diode which converts the input photo current into a logarithmically compressed voltage. The voltage is used as a photometric value for exposure control.
The above mentioned type of light measuring apparatus has the advantage that the responsiveness of the output voltage to the change of light intensity is higher than other conventional system wherein such open voltage is used for exposure control which is generated at the both ends of a photo diode and logarithmically converted according to the intensity of light.
However, the known light measuring apparatus has some difficult problems.
When the supply voltage is applied to the light measuring circuit, there is generated noise in the circuit. Relative to the rise time of the supply voltage, a longer time is required before the operational amplifier gets in a stabilized state. Because of the generated noise and the unstable period of the operational amplifier, some amount of opposite charge is accumulated at the connection point of photo diode and logarithmic conversion diode where the input impedance is extremely high. This accumulation of opposite charge causes the problem of so-called "latch". The latched state remains not lost for a long time. This makes it impossible to obtain a normal measuring voltage from the circuit in a short time after the application of power source to the circuit. In fact, hitherto, it has been required a long time to obtain the normal output voltage from the light measuring circuit.
To solve the problem of latch it has been proposed to provisionally short-circuit the two ends of the photo diode by use of a transistor only during the time of the operational amplifier being unstable. However, according to the solution, there occurs abrupt change of potential at the switching of the transistor. The potential change is transmitted to the connection point through stray capacity as noise which causes latch again. It has been found that the solution is not effective for cancelling the latch and sometimes it is rather harmful for the latch cancellation.
As another solution to the problem of latch it is also known to use a light emitting element such as LED in the circuit to generate a neutralizing current by it only during the unstable period of the operational amplifier thereby cancelling the latch by the generated current. However, this solution has some drawbacks. It needs voluminous and expensive circuit. Further, the power consumption increases up.
The above mentioned problem of latch occurs also in flash light photographing employing TTL flash output control.
To electrically control the high speed shutter of camera there is generally used OFF-type magnet. OFF-type magnet is such type of magnet which is holding the closing blade of shutter during the application of current to the magnet coil and releases the holding of the closing shutter blade when the supply current to the coil is cut off. With this type of magnet it is possible to control a high speed shutter in a stable manner. However, when the supply current to the coil is rapidly cut off, a considerable amount of kick-back noise is produced by inverse induced voltage. In flash light photographing, the operation of shutter is carried out after stopping the flash light emission by the flash output control. This means that the kick-back noise is generated in the dark. In case of a camera for which a high density mounting is required, it is very difficult to prevent TTL metering circuit from being affected by the kick-back noise. In practice, it may be impossible. The light measuring circuit for TTL flash output control is necessarily latched whenever it is affected by such kick-back noise in the dark. When the next shooting with flash light is carried out immediately after one shooting, it is impossible to calculate the correct and proper exposure value for the next shooting unless the latch caused by the kick-back noise generated at the previous shooting is cancelled before the next shooting. The reason for this is that the output of the measuring circuit can not become normal unless the charge which has caused the latch is neutralized. So long as the output is in the abnormal state, any correct value of measured light can not be obtained. In the dark, the current available for the neutralization is substantially zero. Therefore, if two or more flash light exposures are carried out in succession, a large portion of the photo current will be consumed as the neutralizing current to cancel the previously caused latch. As the photo current is partly consumed as the neutralization current, the flash output control is rendered unstable and unreliable. The photo current consumed for the latch cancellation has an important effect on the correctness of measurement in particular when a film of high photosensitivity is used. The higher the photosensitivity of film is, the higher ability is required for the light measuring circuit to measure the light under the condition of less photo current. The error in measurement caused by the photo current consumed for the above neutralization, therefore, increases with increasing the film sensitivity. Consequently, the film which can practically be used is limited to the low sensitivity film only. This means that the range of film sensitivity usable is narrowed, which brings forth a serious problem.